In the manufacture of an electronic device such as a semiconductor device, a plasma processing is performed on a processing target object by a plasma processing apparatus. Generally, the plasma processing apparatus includes a processing vessel, a gas supply system, a first electrode, a second electrode, a first high frequency power supply and a second high frequency power supply. The gas supply system is configured to supply a gas into the processing vessel. The first electrode and the second electrode are disposed such that a space within the processing vessel is provided therebetween. The first high frequency power supply is configured to supply a first high frequency power for plasma generation to the first electrode or the second electrode, and the second high frequency power supply is configured to supply a second high frequency power having a relatively low frequency for ion attraction to the second electrode. Generally, in the plasma processing performed in this plasma processing apparatus, the gas is supplied into the processing vessel from the gas supply system, and the first high frequency power from the first high frequency power supply is supplied to the first electrode or the second electrode to generate plasma. The second high frequency power from the second high frequency power supply is supplied to the second electrode when necessary.
There is a plasma processing in which a first process of generating plasma of a first gas and a second process of generating plasma of a second gas are performed alternately. That is, in this plasma processing, the first gas and the second gas are supplied into the processing vessel alternately, and a first high frequency power is supplied to the first electrode or the second electrode over the first process and the second process to generate plasma. Further, the second high frequency power may be supplied to the second electrode in the second process without being supplied in the first process.
Since a gas has a mass, it takes time for the second gas to actually reach the processing vessel from a time when the gas supply system is controlled to switch the gas supplied into the processing vessel from the first gas to the second gas. Meanwhile, the second high frequency power is supplied to the second electrode in a negligible time length after the second high frequency power supply is controlled to supply the second high frequency power. Accordingly, it may happen that the second high frequency power is supplied to the second electrode at a time when the second gas does not reach the inside of the processing vessel.
In this regard, there is proposed a technique of starting the supply of the second high frequency power after it is determined from a detection result of an emission spectrum within the processing vessel that the second gas has reached the inside of the processing vessel. This technique is described in Patent Document 1.    Patent Document 1: Japanese Patent Laid-open Publication No. 2013-058749
There may be no detectable difference between an emission spectrum of the plasma of the first gas and an emission spectrum of the plasma of the second gas. For example, depending on the kind of a gas contained in the first gas and the kind of a gas contained in the second gas, there may be no detectible difference between the emission spectrum of the plasma of the first gas and the emission spectrum of the plasma of the second gas. In case that there is no detectible discrepancy between the emission spectrums of the plasma of the first gas and the plasma of the second gas, the timing when the second gas has reached the inside of the processing vessel cannot be detected with high accuracy. Resultantly, the supply of the second high frequency power cannot be begun at an appropriate timing.
Therefore, there arises a demand for a technique of accurately detecting the timing when the second gas has reached the inside of the processing vessel in the plasma processing in which the first process of generating the plasma of the first gas within the processing vessel of the plasma processing apparatus and the second process of generating the plasma of the second gas within the corresponding processing vessel are performed alternately.